Synclines and anticlines are series of folds in the Earth's surface with crests and troughs. Anticlines are antiforms that are the downward curving folds that peak at the center and synclines are synforms that are the upward curving folds that trough at the center. They are caused by lateral compression forces causing sedimentary layers to buckle under the stress. It is important to note that both formations are usually formed together as alternating ridges; one is not usually naturally favored over the other. So an anticline forms because it is flanked by synclines and the only room for the layers to move is up. Compressional forces do not have to form synforms and antiforms. Domes and basins can form as well. If a fault or break is present at the site of the force, other structures can form like mountains.